1. Field of the Invention
The present invention relates to a dehydrating device with controlled water absorption. More particularly, it relates mainly to a sheet for causing dehydration by contact with a food; the sheet able to be applied in a variety of fields such as a pretreatment of, for example, drying or freezing fish, meat, vegetable or the like, food preservation, food processing, cooking etc.
2. Description of the Related Art
A variety of contact dehydrating sheets which comprise a combination of a semipermeable membrane and a high osmotic pressure substance, and utilize a difference in osmotic pressure, are proposed, for example, in Japanese Examined Patent Publication No. 58124/83, and U.S. Pat. Nos. 4,383,376, 3,645,698 and 4,686,776. All of these devices contain a polymeric water absorber therein, and thus the high osmotic pressure substance absorbs water through the semipermeable membrane, and the water moves from the high osmotic pressure substance to the polymeric water absorber and is fixed therein. The polymeric water absorber has a high water absorbing capacity of as much as several tens of times to several hundreds of times its weight when empty, and can maintain the water absorbing function for a long period; that is, can retain a high osmotic pressure, and show a high water absorbing capacity.
Conventional dehydrating sheets containing a polymeric water absorber, have a long term retention of water absorbing function, and thus may be often inconvenient, depending on the intended use. In other words, to control the water absorption to an appropriate level, it is necessary to remove the dehydrating sheet at the proper time.
A dehydrating sheet is required which, upon having absorbed a desired amount of water, will absorb substantially no more water because of a reduction of dehydrating capacity, and thus need not be removed. In addition, the water absorbing function of a polymeric water absorber will be greatly reduced when absorbing water in which ionic substances such as Ca.sup.++, Mg.sup.++, Na.sup.+, K.sup.+, Cl.sup.- and the like coexist. Accordingly, the water absorbing function can be varied depending on the purpose therefor, and it may be difficult to control the dehydration.